1. Field of the Invention
The present invention relates to an electron microscope e.g, a scanning electron microscope or a transmission electron microscope.
2. Summary of the Prior Art
An electron microscope normally comprises a specimen chamber in which the specimen to be analysed is located, and an electron column which generates a beam of electrons which are used to bombard the specimen. That electron column itself normally has two parts, an electron gun chamber containing an electron gun for generating the beam of electrons, and a casing containing an electron lens arrangement. That electron lens arrangement usually includes a plurality of condenser lenses arranged vertically inside the casing with the electron beam passing down a central axis of those condenser lenses. Alignment coils for aligning the electrons into a narrow beam are located between the condenser and the electron gun. Finally, in e.g, a scanning electron microscope, scanning coils (deflection coils) are located between the condenser lenses and the sample, which cause the beam to scan across that sample. Thus, an electron beam path is defined between the electron gun and the sample, passing through the lenses and coils.
It is important that electrons following that path are not scattered, and for this reason the interior of the electron column has to be evacuated to a low pressure. The sample chamber must also be evacuated. The standard method of evacuating the interior of the electron column is to connect that column to a vacuum pump via a plurality of ducts which extend from the vacuum pump through the walls of the electron column. Example of such an arrangement is shown in Japanese patent application laid-open number 49-131376, in which there are three ducts extending from the electron column and one from the sample chamber which are interconnected at a diffusion pump. Similarly, in Japanese patent application laid-open number 55-136446, there are again a plurality of ducts leading from the electron column.
However, the presence of such ducts causes a number of problems. The first problem is that, if a good vacuum is to be achieved within the electron column, it is necessary that the ducts be relatively large, so that they have satisfactory conductance. The size of the ducts is inconvenient, and limits the positioning of additional devices which may be heeded around the electron column, such as an X-ray system.
A further problem associated with the presence of the ducts to the vacuum pump is that their presence means that apertures must be made in any magnetic shielding around the electron column. Stray magnetic fields may cause magnetic disturbance, and if this is allowed to affect the electron beam, the accuracy of the operation of the microscope may be compromised. For example, a magnetic disturbance due to e.g. noise may cause a ripple on the image produced by the electron microscope, thereby blurring that image. Therefore, in order to limit magnetic disturbance. it is known to place a shielding cylinder around the electron column, but it is necessary that apertures be made in that shielding to permit the ducting to the vacuum pump to emerge. Bearing in mind that that ducting needs to be large in order to achieve a good vacuum, there is thus a conflict between the need for a good vacuum and the need for good magnetic shielding.